As the frequency of operation of CCD sensors increases, and the system signal to noise requirements increase, it becomes increasingly important to provide good electrical contact to the CCD imager substrate. In addition, CCD sensors have relatively high capacitance loading on their high speed clocks. This compounds the problem, as the CCD is clocked at higher rates, more power is dissipated (creating heat) and this increases the need for good heat dissipation. (Dark current, a thermally generated noise source, doubles with every 10 degree Celsius substrate temperature increase.) CCD imagers have a transparent window on their front surface, therefore any heat sink attachment must be done on the rear surface. Linear CCD imagers compound the noise problem by virtue of their increased length. The increased length increases the length of the current return paths to the CCD support electronics.
The mounting of the linear type CCD array therefore should be such that minimal noise is generated and heat dissipation is efficient.
CCD sensors work by applying various clock and bias voltages to the device, the bias voltages being measured against a reference potential on the sensors. The reference plane on the sensor can be either the substrate of the device itself or a well potential if the imager is built within a P or N type well. Clock drivers are designed to supply high edge currents in order to provide the changes in voltage required by the sensor. Typical voltage drive waveforms are square waves. When driving a capacitive load the primary current flows on the transitions from one voltage level to another. The current must flow out of the driver through the capacitive load to the sensor's reference plane. All non-cancelled currents are returned to the drivers thereby closing the loop. Resistance or inductance in this current path results in a voltage drop being generated and will cause the reference plane of the sensor to "bounce" on clock edge transitions generating noise in the imager output. Reducing the parasitic inductance and resistance of the current return path is critical to the reduction of clock feed-through generated noise.